I have an old fence that leans too much as the concrete post supports have started to tilt towards the adjacent slope over the years. I'm considering if there are new post brackets that can be adjusted in angle so I can use the leaning supports. Alternatively, new post brackets that can be shimmed in some way.
There is probably no shortcut for you; the plinths will tilt/give way over time even more outward. If they are on the edge of the slope, the risk is quite high. It will be a lot of work; you must dig and straighten the plinths and ensure they are firm around them. Possibly even move them in at least 30 cm to keep them standing. Too close to the slope is harmful.
EDIT It might be possible to drive down 2 rebar rods 12 mm at least about half a meter (preferably deeper) and go up as far as the plinth, as support on the outside against the slope, but you must straighten them. No guarantee on the last part, but it might work.
EDIT It might be possible to drive down 2 rebar rods 12 mm at least about half a meter (preferably deeper) and go up as far as the plinth, as support on the outside against the slope, but you must straighten them. No guarantee on the last part, but it might work.